That’s 4 million in 3 days. Solid, right? Sure. Though it has to
be noted that Apple sold 3 million copies of OS X Mountain Lion in
4 days. Microsoft likes to poke fun at the small OS X install
base, and now it works both ways. Apple got nearly as many people
to buy the update for OS X in the same amount of time despite a
sliver of the overall footprint.

Microsoft’s has an installed base of 1.25 billion users. According
to Ballmer’s own numbers, 4 million upgraded to Windows 8 in the
first four days. That means that one-third of one percent of
Microsoft’s user base upgraded.

Now let’s take a look at Apple. In its first four days on sale,
Mountain Lion sold 3 million copies. Apple has an installed base
of 66 million, meaning that 5 percent of Apple’s user base updated
in four days.

This is a really hard comparison to make, though. It speaks to the differences in Apple’s and Microsoft’s customers more than it does the differences between Windows 8 and Mac OS X.